Deep within the earth, flow of molten iron generates electric currents which in turn produce magnetic field which protects the earth from devastating solar winds. According to some estimates, the Earth's magnetic field has weakened by 15 percent over the last 200 years with additional evidence emerging that the weakening is happening unevenly with some areas across the planet getting more protection. According to data collected by a European Space Agency (ESA) satellite array called Swarm, the biggest weak spots in magnetic field have sprung up over the Western Hemisphere and strengthened over areas like the southern Indian Ocean. It has been suggested that powerful electric currents are generated deep inside the Earth causing low frequency electromagnetic signals that have long been reported in connection with impending earthquake activity. The current from the stressed rocks is carried out to and through unstressed surrounding rocks by positive holes similar to related ones in semiconductor materials. These underground positive holes leave behind a surplus of electrons. This process gives rise to positive air ions which changes local and even regional magnetic fields and could be an indication of an impending earthquake in the area. Such low-frequency electromagnetic emissions (EM) have been documented by a large body of satellite data and/or ground-based data.
Generally, critical scientific data to study electromagnetic field strengths indicative of geophysical weather and atmospheric changes lies in a frequency response range from DC to 100 Hz. Accordingly, there is a need for a network of ground-based magnetometers to track and record minute changes in various measurements, e.g., Schumann resonance variation, Earth's static magnetic field variation, and/or the planetary static and dynamic magnetic fields with sensitivity from the nano-tesla to pico-tesla range.
In 1952, Schumann published a paper about standing electromagnetic waves in the waveguide between the Earth's surface and the ionosphere. These waves are known as Schumann resonance (SR) waves and can be used for various scientific studies ranging from global lightning to detection of space weather and global climate variations. The Schumann resonances are a set of resonant modes or spectrum peaks, between 7.83 and 45 Hz. These waves are very weak compared to the Earth's much larger static geomagnetic field, which is on the order of 50,000 micro-gauss. Solar or geomagnetic activity can be tracked by measurement of changes of the dielectric permeability in the Schumann cavity. For example, lightning is one such natural phenomenon and can be tracked around the clock by measurement of Schumann resonance values. The Schumann resonances offer means for investigating tropospheric-ionospheric coupling mechanisms related to lightning activity and wave propagation in the ionosphere. At any given moment, there are between 50 and 100 lightning flashes around the globe. These lightning flashes create low frequency electromagnetic waves which are trapped between the ionosphere and the Earth.
Extremely low frequency (ELF) electromagnetic waves have long wavelengths. For example, 10 Hz corresponds to a wavelength of 30,000 kilometers. In atmospheric and magnetosphere science, the lower frequency electromagnetic oscillations are considered to lie in the extra low frequency (“ELF”) range. It is extremely difficult to build an antenna to capture these ELF signals. The ELF frequencies have been used in only a very few man-made communication systems. Due to long electric power lines, there are unintentional sources of ELF radiation, in the 50 or 60 Hz range. Due to their long wavelength, these ELF waves can penetrate seawater and significant distances down into earth or rocks, and through the earth. Similarly if ELF electromagnetic waves are generated deep inside the earth, these signals are capable of appearing on the surface and can be detected using low frequency electromagnetic antennas or magnetometers. One of the requirements of these ELF antennas or magnetometers is that they should produce extremely low background noise and enough sensitivity to detect weak signals. Electronic l/f noise is another factor which has to be considered when designing ELF antennas or low frequency magnetometers.
One of the other sensing systems to detect magnetic fields is a standard LC oscillator circuit where L is made of a coil, wound on a typical magnetic material. The magnetic material coil L becomes part of the oscillator LC circuit to sense the surrounding magnetic field. Because a resonant frequency varies as the square root of magnetic field strength in a standard LC oscillator circuit, a measured shift in frequency of an LC oscillator circuit provides an indication of a relative magnetic field strength, with decreasing precision at lower frequency responses. Thus, LC oscillator circuits are sufficient designs for Gaussmeters, i.e., for measuring larger scale magnetic fields. In some other magnetic field measurement circuits, relaxation oscillators are employed which produce a non-sinusoidal repetitive output signal. These circuits have a resonant frequency that varies directly with magnetic field strength, thus making them a more preferred design choice for measuring low frequency signals. However, relaxation oscillators have stability issues. In relaxation oscillator circuits, the electronic device operates in an extremely nonlinear fashion and any variation in oscillation amplitude may also appear as a frequency change. They have low duty cycle and poor phase-noise as well.
Accordingly, there are difficulties in designing sensor systems capable of measuring magnetic fields of low level strength and for extreme low frequency measurements that overcome the aforementioned drawbacks. For example, flux gates sensors, which employ coils of wire around a core of highly permeable magnetic material to directly sense a magnetic field, are difficult to build and require low noise readout circuitry. Super Conducting Quantum Interference Devices (SQUIDS) can measure subtle magnetic field strengths using superconducting loops containing Josephson junctions, but require cryogenic cooling and are therefore unsuitable for most field applications.
Search coil magnetometers use coils around a high permeability core to measure variation in magnetic flux, but generally have poor sensitivity at low frequencies. Certain search coil magnetometers have been developed for measuring low magnetic field strength signals which employ ferromagnetic or μ-metal materials but require large number of turns, are bulky, and suffer from eddy and damping losses. Because ferromagnetic and μ-metal based sensors have relatively constant permeabilities, a time rate change of field (dH/dt) is used to measure ambient magnetic field. However, a time rate change of field at low frequencies is small, thus making signal detection at such low frequencies very difficult. Further, the calibration of p-metal or ferromagnetic material magnetometers is difficult.
For example, a search coil magnetometer made of p-material has been installed in the Antarctic region of the Southern hemisphere to study waves and transient variations in Earth's magnetic field in the Ultra-Low-Frequency range, from approximately 0.001 Hz to 5 Hz. This magnetometer has 160,000 turn coils of copper wire mounted on 2.625-foot long rod which weighs more than 15 pounds. This magnetometer is not suitable for frequency response above 5 Hz and is therefore not an appropriate sensor for measuring Schumann resonance frequency ranges. Further, due to its conductive nature, this material can cause damping losses. Similarly another search coil magnetometer, installed on the NASA THEMIS mission, has 51,600 turns and requires another secondary winding to introduce a flux feedback in order to flatten the frequency response. This magnetometer suffers from low sensitivity at higher frequency as well.
As described above, most known magnetometers used for aerospace applications were developed using magnetic materials having relatively constant permeabilities. A variable permeability magnetometer takes advantage of a variance of permeability of a magnetic material with a changing in ambient magnetic field. Known sensors using the variation of the permeability of a magnetic core are able to measure large magnetic fields on the order of one Gauss, e.g., Gaussmeters. For example, U.S. Pat. No. 4,851,775 is directed to a magnetometer which includes a sensor coil around a strip of Metglas Amorphous Alloy 2705 M in a relaxation oscillator circuit. However, as previously described, relaxation oscillator circuits suffer from stability problems, so they are not suitable for precision measurements of low strength magnetic fields.
Prior solutions for measuring magnetic field strengths have not resolved the need for an approach to measuring frequency response in a Schumann resonance range while still employing a relatively simple and compact construction. Therefore, there is a need for systems and methods that address one or more of the deficiencies described above.